AdBlue and NOx sensor faults

Part 1

AdBlue and NOx Sensor Faults Explained

SCR uses AdBlue dosing to help reduce nitrogen oxides in the exhaust. NOx sensors measure nitrogen oxide levels; depending on the system, sensors before and after the SCR catalyst support dosing control and efficiency monitoring. A warning may result from a sensor fault, wiring damage, communication loss, incorrect dosing or poor SCR performance. Some faults can trigger a restart-inhibition countdown, but the warning sequence varies by vehicle. These generic code descriptions are starting points, not component diagnoses; manufacturer-specific subcodes also matter.

• P20EE: SCR NOx catalyst efficiency below threshold, bank 1.
• P208E: Reductant injection valve stuck closed, bank 1 unit 1; this does not by itself prove a blockage.
• P229F: NOx sensor circuit range/performance, bank 1 sensor 2.
• U029E: Lost communication with NOx sensor B.
• P207F: Reductant quality performance; this does not by itself prove contaminated AdBlue.

Part 2

Diagnostic Checks and Vehicle Fitment

SCR is fitted to many UK diesel cars and vans, including relevant Mercedes-Benz BlueTEC, Citroën and Peugeot BlueHDi, and Land Rover Ingenium diesel variants. Fitment and sensor layout depend on the exact model, engine, build date and emissions specification; the badge or engine family alone is not enough. SCR-equipped vehicles are not confined to the 2015 to 2023 period.

• Read all relevant control modules and record fault subcodes and freeze-frame data before clearing anything.
• Check sensor power supplies, earths, wiring, connectors and communication where indicated.
• Assess NOx readings under suitable operating conditions rather than condemning a sensor from one code.
• Check dosing pressure, injector operation, exhaust leaks and exhaust temperatures as the fault requires.
• Check fluid condition and applicable manufacturer software guidance where relevant.

Part 3

Repair and Software Changes Are Different

A repair should address a demonstrated cause rather than rely on trial-and-error replacement. Manufacturer software updates or guided resets may form part of a repair where applicable, but they are not the same as disabling SCR monitoring. A bypass does not restore dosing, sensor accuracy or catalyst performance. After repair, appropriate functional tests and any required adaptations should confirm whether the system is working correctly.

Part 4

Mobile Assessment and Restart Warnings

Send your current postcode and describe where the vehicle is parked so we can confirm coverage, access and whether mobile assessment is suitable. Include whether it starts and the exact wording of any countdown. Some faults need workshop facilities or further testing, so roadside completion and avoidance of recovery cannot be promised. If the vehicle displays a restart restriction, follow its handbook guidance and seek an assessment promptly.

Part 5

Information to Send Before Assessment

A few details help distinguish a fluid-level warning from a dosing, sensor or communication fault. Please provide whatever information you have; do not replace parts solely on the basis of a generic code description.

• Vehicle registration and mileage.
• Current postcode and parking or access details.
• A clear photo of the complete dashboard message.
• Fault codes, including manufacturer-specific subcodes if available.
• Recent AdBlue top-ups, repairs or software work.
• Whether the warning is intermittent, a countdown is active or the engine will not restart.